Thermal inkjet printheads have two major issues that are generally lumped in the category called printhead maintenance. The two issues are coined “startup” and “idle time.” When a printer is not in use for any time period from a few minutes to a few weeks, the printhead is generally capped to prevent excessive drying of ink in the nozzles which are exposed to the air. Evaporation of water from the ink in the nozzles causes the viscosity to increase significantly. Still, no cap can provide a perfect seal, and drying occurs either due to leakage in the seal, or diffusion through the capping materials. For typical extended capping periods in the range of a weekend, the drying can be excessive enough to require large amounts of waste “spits” before starting up the first printing job. Even worse, at times a pump must be employed to pull ink through the nozzles to eliminate the dried ink. This is the essence of the “startup” problem. To get a nozzle with dried ink to startup, a certain amount of ink must be wasted prior to the nozzles first print on the page. In addition to the wasted ink, these startup algorithms are often noisy, and can take significant time—sometimes several minutes—which is undesirable to the customer, because it increases the time to first print.
While printing, and depending on the images being printed, many nozzles may not be called upon to produce a dot. In this case, idle nozzles aren't capped, but rather they are in open air and they experience forced convection due to the movement of the printhead carrier. If a nozzle hasn't been used for as little as a few seconds during a print job, then the ink in the nozzle can dry, and the viscosity can increase enough to compromise the quality of the next few ejected drops from that nozzle. To avoid noticeable print quality issues due to idle time, typically the printhead returns to the maintenance station to spit all nozzles at a required interval. Again, this wastes ink, passing undesirable added cost to the customer. Since nozzles that are uncapped and on a moving printhead become unacceptably dry in a matter of a few seconds, idle time spits can have a significant, adverse effect on print speed.
Waste ink generated by idle time and startup spits increases linearly with the number of nozzles present in the inkjet printhead. Thus, as heater chips continue to grow in length to increase print speed the waste ink amount grows to uneconomical volumes, in some cases equaling or even exceeding the amount of usable ink in the system. In the case of a page-wide printhead, it is extremely so. Also, for specialty inks with higher solid content in the commercial/industrial printing markets, such as a latex ink or ink with binder, this problem gets increasingly worse. In applications where maintenance stations do not exist, such as in many industrial printing applications, there is also a need for the printhead to startup without any maintenance spits.
Other solutions to these two problems involve re-circulating ink upstream of the ejector, or often even upstream of the filter. However, to truly keep nozzles fresh and ready to fire, recirculation must happen at the ejector level. Another conventional solution is to recycle ink that has been primed through the nozzles for the purpose of enabling a quality startup. Although this technique avoids wasting ink, in a multi-color printer it requires a separate seal for each color thereby rendering the technique nearly impossible to use with a multi-color heater chip. Also, this technique can be used to reclaim ink from mid-print job spits for idle time, but it is not helpful for increasing the time interval required between spits. Thus, the print speed reduction due to idle time spits is not reduced.
Other conventional art includes United States Patent Application US2012/07921A1. Here, an extra heater positioned at the end of the heater array is used to pump ink from the via, along the array, through the ejectors and back into the via. This embodiment has a significant drawback in that a pressure drop occurs across each ejector resulting in minimal flow rate toward the center of an array of ejectors.
A solution is needed for the startup and idle time problems that doesn't waste ink, works for a multi-color heater chip, and does not slow down print speed.